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.TH MCHECK 3  2021-03-22 "GNU" "Linux Programmer's Manual"
.SH NAME
mcheck, mcheck_check_all, mcheck_pedantic, mprobe \- heap consistency checking
.SH SYNOPSIS
.nf
.B #include <mcheck.h>
.PP
.BI "int mcheck(void (*" abortfunc ")(enum mcheck_status " mstatus ));
.BI "int mcheck_pedantic(void (*" abortfunc ")(enum mcheck_status " mstatus ));
.B void mcheck_check_all(void);
.PP
.BI "enum mcheck_status mprobe(void *" ptr );
.fi
.SH DESCRIPTION
The
.BR mcheck ()
function installs a set of debugging hooks for the
.BR malloc (3)
family of memory-allocation functions.
These hooks cause certain consistency checks to be performed
on the state of the heap.
The checks can detect application errors such as freeing a block of memory
more than once or corrupting the bookkeeping data structures
that immediately precede a block of allocated memory.
.PP
To be effective, the
.BR mcheck ()
function must be called before the first call to
.BR malloc (3)
or a related function.
In cases where this is difficult to ensure, linking the program with
.IR \-lmcheck
inserts an implicit call to
.BR mcheck ()
(with a NULL argument)
before the first call to a memory-allocation function.
.PP
The
.BR mcheck_pedantic ()
function is similar to
.BR mcheck (),
but performs checks on all allocated blocks whenever
one of the memory-allocation functions is called.
This can be very slow!
.PP
The
.BR mcheck_check_all ()
function causes an immediate check on all allocated blocks.
This call is effective only if
.BR mcheck ()
is called beforehand.
.PP
If the system detects an inconsistency in the heap,
the caller-supplied function pointed to by
.I abortfunc
is invoked with a single argument,
.IR mstatus ,
that indicates what type of inconsistency was detected.
If
.I abortfunc
is NULL, a default function prints an error message on
.IR stderr
and calls
.BR abort (3).
.PP
The
.BR mprobe ()
function performs a consistency check on
the block of allocated memory pointed to by
.IR ptr .
The
.BR mcheck ()
function should be called beforehand (otherwise
.BR mprobe ()
returns
.BR MCHECK_DISABLED ).
.PP
The following list describes the values returned by
.BR mprobe ()
or passed as the
.I mstatus
argument when
.I abortfunc
is invoked:
.TP
.BR MCHECK_DISABLED " (" mprobe "() only)"
.BR mcheck ()
was not called before the first memory allocation function was called.
Consistency checking is not possible.
.TP
.BR MCHECK_OK " (" mprobe "() only)"
No inconsistency detected.
.TP
.B MCHECK_HEAD
Memory preceding an allocated block was clobbered.
.TP
.B MCHECK_TAIL
Memory following an allocated block was clobbered.
.TP
.B
MCHECK_FREE
A block of memory was freed twice.
.SH RETURN VALUE
.BR mcheck ()
and
.BR mcheck_pedantic ()
return 0 on success, or \-1 on error.
.SH VERSIONS
The
.BR mcheck_pedantic ()
and
.BR mcheck_check_all ()
functions are available since glibc 2.2.
The
.BR mcheck ()
and
.BR mprobe ()
functions are present since at least glibc 2.0
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR mcheck (),
.BR mcheck_pedantic (),
.BR mcheck_check_all (),
.BR mprobe ()
T}	Thread safety	T{
MT-Unsafe race:mcheck
const:malloc_hooks
T}
.TE
.hy
.ad
.sp 1
.SH CONFORMING TO
These functions are GNU extensions.
.SH NOTES
Linking a program with
.I \-lmcheck
and using the
.B MALLOC_CHECK_
environment variable (described in
.BR mallopt (3))
cause the same kinds of errors to be detected.
But, using
.B MALLOC_CHECK_
does not require the application to be relinked.
.\" But is MALLOC_CHECK_ slower?
.SH EXAMPLES
The program below calls
.BR mcheck ()
with a NULL argument and then frees the same block of memory twice.
The following shell session demonstrates what happens
when running the program:
.PP
.in +4n
.EX
.RB "$" " ./a.out"
About to free

About to free a second time
block freed twice
Aborted (core dumped)
.EE
.in
.SS Program source
\&
.EX
#include <stdlib.h>
#include <stdio.h>
#include <mcheck.h>

int
main(int argc, char *argv[])
{
    char *p;

    if (mcheck(NULL) != 0) {
        fprintf(stderr, "mcheck() failed\en");

        exit(EXIT_FAILURE);
    }

    p = malloc(1000);

    fprintf(stderr, "About to free\en");
    free(p);
    fprintf(stderr, "\enAbout to free a second time\en");
    free(p);

    exit(EXIT_SUCCESS);
}
.EE
.SH SEE ALSO
.BR malloc (3),
.BR mallopt (3),
.BR mtrace (3)
